Electrical wave transmission system



Oct. 19, 1937. P. 0. FARNHAM 2,096,393

ELECTRICAL WAVE TRANSMISSION SYSTEM Filed March 21, 1931 2 Sheets-Sheet l Eda? akin/way.

Oct. 19, 1937. P. o. FARNHAM I ELECTRICAL WAVE TRANSMISSION SYSTEM Filed March 21, 1951 2 Sheets-Sheet 2 IOOOO moo 140/5004 02 00) in M crovo/fs Jwmzntot i @MWZW,

Patented Oc 19, 1,937

, ELECTRICAL WAVE ,TRANSMISSION SYSTEM Paul 0. Farnham, Boonton, N. J., assignor, by meene assignments, to Radio Corporation of America, New York, N. Y., a corporation of Delaware Application March 21, 1931, Serial No. 524,383

20 Claims.

This invention relates to electrical wave transmission systems, and particularly to systems employing diode rectifiers for demodulatingcarrier ,;'waves and/or for automatically regulating the 5 gain of a vacuum tube amplifier.

, It has been recognized that a diode rectifier may be so operated as to develop a direct current potential substantially proportional to the impressed carrier voltage, and use has been made of this property to control the gain of an amplifier which precedes the diode rectifier.

Previous systems in which a diode detector has been used to furnish a direct current potential for the purpose of controlling amplification in the preceding stages as well as to furnish an audio output for further amplification have been open toseveral objections. Satisfactory gain and selectivity from a circuit feeding the detector were difficult to obtain when the diode biasing resistor 2 was in shunt with the input circuit. Since the direct current 'potential for automatic volume control purposes was developed at low carrier levels on the detector the characteristic of the F, control suffered. That is, the action of the autofmatic Volume control with increasing input to the receiver wasnot sharpand did not hold the audio output substantially constant with increasing inputs. A further objection to previousautomatic H gaincontrol systems has been that while thereceiver was tuned to frequencies at which no strong In the accompanying drawings, which illustrate I certain specific embodiments of the invention,

Fig. 1 is a circuit diagram of a carrier wave re- .ceiver including a diode rectifier,

Fig. 2 is a curve sheet illustrating the relation- 7 ship which may obtain between carrier input and rectified output in a rectifier such as shown in Fig. 1,

1. I Fig. 3 is a fragmentary circuit diagram of an- 1 '5 other embodiment of the invention, and

Fig. i is a curve sheet illustrating the relationship between the signal input to a receiver such as shown in Fig. 1, and the audio output of the receiver.

In Fig. l the reference numeral I identifies the collecting structure which passes received carrier wave signals to the tuned input circuit, comprising inductance 2 and variable condenser 3, of a radio frequency amplifier tube 4. The amplified 60 output from 'tube 4 is passed to a network, indicated generically at 5, which may comprise a simple coupling impedance or may include additional amplifier stages.

The input impedance of the diode rectifier is coupled to the output terminals of the network 5 and, as illustrated in Fig. 1, the input impedance comprises an inductance 6 in series with a tuning condenser l and blocking condenser 8, the junction of the condensers being connected to ground and to the low potential output terminal of the network 5. The coupling to the network 5 may be of any appropriate type, a capacitive coupling through condenser 9 being illustrated as typical of one form.

As stated in the copending application of Knox Charlton Black, Ser. No. 446,755, filed April 23, 1930, patented July 26, 1932, as U. S. Patent 1,869,089, any tube element in the electron field from a cathode will, irrespective of the operation of the other tube elements, give satisfactory diode rectification in conjunction with the cathode, provided a small positive direct current voltage will cause appreciable current to flow to the electrode. According to the present invention, this principle of vacuum tube operation is utilized by employing two or more tube elements as the anodes of substantially independent diode output circuits. With this arrangement it becomes possible to exercise a selective control over the development of each of the rectified voltages which are to be obtained from the rectifiers. Furthermore, to simplify the construction of the output circuits, the diode connections are inverted, i. e., the high carrier potential terminal of the input circuit is connected to the diode cathode.

The multiple diode rectifier IE9 may be, and preferably is, a commercial triode of the type having a heater H, an equipotential cathode 42, grid l3 and plate I4. For convenience of description, the electrode i3 will be referred to as the grid, but it will be apparent from the following description that the grid !3 functions as the anode of a diode rectifier. The high potential side of the tuned input circuit is connected to the cathode II and, so far as concerns carrier frequencies, the input circuit is completed to the rectifier anodes by the radio frequency by-pass condenser l5 which is connected between the grounded terminal of the input circuit and the grid, and the audio frequency by-pass condenser l6 which is shunted across the grid and plate terminals. I

The audio frequency output voltage is developed across resistive impedance in the circuit between the grid [3 and cathode I2. The low carrier potential terminal of the inductance 6 is connected to ground through a resistance IT and the audio output circuit of the rectifier includes the resistance l8, which, by an adjustable tap l9, may be connected to any desired point along the resistance H. The direct current output resistance 28 is connected between plate l4 and the grounded terminal of the resistance ll.

The opposite terminal of resistance I7 is connected by a lead ii to a source of direct current potential which is positive with respect to ground, thus providing a drop of potential along resistance l! which applies bias voltages E1, E2 upon the anodes l3, M, respectively, which are negative with respect to the cathode voltage.

The audio frequency voltage may be transferred to the audio amplifier through the block ing condenser 22 and voltage divider 23, the tap on the voltage divider being adjustable to regulate the output level of the receiver. The direct current voltage developed across resistance 29 may be returned to the amplifier tube t as an automatic gain control bias, the lead 24 from the diode plate M to the low potential terminal of the inductance 2 including a filter 25 for suppressing alternating components. The low potential terminal of inductance 2 is connected to the tuning condenser 3 through the blocking condenser 26, and the cathode circuit of the amplifier includes a bias resistor 2'! for supplying the normal, or maximum sensitivity, bias to the amplifier grid.

As now customary, the several tuning elements of the receiver, 1. e., the condensers 3 and l, and any tuning elements which may be included in the network 5 are preferably connected mechanically for simultaneous adjustment, as indicated by the dotted line 23. g

It is to be noted that the particular location @of the audiofrequency by-pass condenser between the grid l3 and plate I4 is important in reducing the distortion in the audio frequency output voltage impressed across the impedance 23 when the impressed signal across the input to the rectifier is just sufiicientto establish a direct current voltage across the resistance 29. Resistance it should preferably be made smaller than resistance 20 for the same purpose.

The operation of-the receiver, and particularly that of the rectifier system, will be apparent from a consideration of the curves of Fig. 2. In the usual diode rectifier systems, no provision is made for applying steady bias potentials to the electrodes anda small flow of direct current takes place even in the absence of any alternating current input.

Curve A shows the relation which would obtain between direct current output and carrier wave input voltage in the absence of a bias potential E2. Similarly curve B illustrates the relationship between audio frequency output Voltage and carrier input voltage in the absence of a bias voltage E1. Upon the application of the initial bias voltages, rectification in the respective output circuits is postponed until the peak carrier voltage impressed upon the rectifier is approximately equal to the respective bias potentials. The relationshipsbetween carrier voltage and the direct current and audio frequency output voltages developed in the circuit of Fig 1 are shown by curves A and B, respectively.

By suitable choice of the bias E2, the development of a direct current .output may be postponed until the signal energy available at the collector structure is suificient, with maximum amplifier gain, to bring the carrier input tothe rectifier up to the value corresponding to the predetermined maximum audio frequency output level.

Whilethis postponement of rectified direct current output maybe useful in various circuit" arrangements, it is particularly effective in automatic gain control systems of thetype represented in Fig. 1. By delaying the development of. a rectified carrier voltage until the signal input reaches a definite value, the receiver sensitivity remains unchanged for all relatively weak signals but, with increasing signal strength, the

gain control voltage'developed by the rectifier becomes effective to hold the rectifier input voltage substantially constant. 7

Due to the high amplification usually obtained in receivers of the automatic gain control type,

the output obtained when the amplifier is tuned to a frequency at which no, or only a'weak, signal input is present is usually accompanied by a'relatively high noise output arising in the tubes and circuits as well as static disturbances. .By-

adjusting the diode bias potential E1 to suppress all audio output for carrier voltages below a magnitude necessary to. give an audio signal free from noise disturbances under normal operating conditions, this noisy audio frequency output can be eliminated.

VTnen the direct current source that supplies current to the resistor ii is not sufficiently free from alternating components, as might'be the case when the direct current is obtained by rectification of alternating current from the house lighting system, it is possible that-the circuit shown in Fig. 1 will impress an objectionable ripple voltage upon the grid of the first audio amplifier. This objection may be overcome, as shown diagrammatically in Fig. 3, by connecting the cathode of the amplifier to such a point on resistance ll that the ripple voltage drops to negligible magnitude.

As illustrated in Fig. 3, the lead 29 from the cathode of the first audio amplifier 3c is not so connected as to eliminate all ripple voltage, but

is connected to the high (direct current) terminal of resistance ii. The remaining elements of the circuit may be substantially identical with the corresponding elements of Fig. 1,' and are identificd by the same reference numerals. While this arrangement does impress upon the first audio amplifier the alternating current component present in the relatively low direct current voltage E1, this ripple voltage will usually be too small to create any disturbing effect when the rectified power supply has been filtered to a degree satisfactory for energization of the plate circuits of the several tubes of the receiver. The arrangement has the decided advantage that the plate voltage applied to the audio amplifier may be so chosen that the direct current drop E1 furnishes an appropriate potential for the audio amplifier. V

Although the invention is not limited to any particular circuit constants, the following tabulation is cited as illustrative of values which have given satisfactory performance.

Certain advantages obtainable by use of the invention will be apparent from a consideration of the curves of Fig.4. The relation betw'een seriously affected .forweak signals and that the audio output continues :to rise a with Y increasing input s'ignal strength even when theimaximum possible .c'on'trol is'eifectivel;

Vifhenslthe operation ofxthe automatic gain control is deferred by the application ofariinitial bias voltage E2, thereisv no reduction in sen'sitivity foroweak'is'ignals and the'relationship .between; audio i'outputand signal input; is .repre sented by "the curve D. The data from which this. :curve was plotted'was obtained when the 7 circuit elements of Fig. l were-not'adjusted to causeany postponement-of audio output. The

operation resultingwhenjan initial 'bias Er: is

applied 'inthe audioloutput circuitof the double diodeisrepresented by; curve F. Theshaded area tothe left of the lower branch of curve F indicates thel range of input' signal strength for which-disturbing noises will mask the audio signal or will bepresent to, such an extent that satisfactory reproduction is not possible. ,1 By

providing a-sharp cut-off at this lower range of input voltages, these noises will'be completely suppressed. and a constantaudio output level will obtainfor substantially allsignals. 1

1 ,So far as concerns the presentinvention, it is tobeunderstoo'd thatthe illustration of a single stageof tuned-radio frequency amplification in Fig. -l isbut illustrative of'o-ne type of circuit with which the invention may be employed,

The principlesas; above outlinedmay, of course, be employed in connection with any of the known automatic: gain controlcircuits such; for exanrple as c ascaded stagesof radio or intermediate frequency amplification orthe first detector in.

the case of a superh eterodyne receiven, Furthermore,;. the multiple-diode rectifier, may bezern q ployedlfor other purposes and with circuits other than thoseof automatic gaincontrol systems; While the invention permits the use of a tunable input circuit for the rectifier, it is possible:

to employ an untuned input impedance, The customaryarrangement in which the low potential sideof the input circuit is connected to the rectifier cathode may be employed byproviding an appropriate by-pass system in thecon nections from the high potential side of the input circuit tothe rectifier'anodes. j 3

,I claim:

I A combined demodulator and carriervoltage rectifier, comprising a double diode having a cathode and two anodes; an input impedance comprising an inductance shunted by a tuning condenser iniseries with a blocking condenser, a

connection from, the junction of-the inductance and-tuning condenser to the rectifier cathode,

aiconnection from the junction of thecondensers to ground, and resistive output impedances con- 7 nected between the respective anodes and ground, capacity by-passing one output impedance for bias potentials between the rectifier elements.

2. Iheinvention as set forth in claim 1 wherein the ,capacity whiehby-passes one output im- 'pedance'ior, audio frequency currents ,is con- 75 nectedac'r oss th e two; anodescf saidrectifier. 5

In an alternating current amplifier, the combination with a vacuum tube amplifier, input and output circuits therefor, and means normally biasing said amplifier to secure a desired amplification of received alternating currents, of a diode rectifier, an input impedance coupled to the amplifier output circuit and effectively connected between the rectifier cathode and anode, an output resistance effectively between the rectifier cathode and anode, a'second diode rectifier, coupledzto'said input'impedance, and including an audio frequency load in its output, means preventing the development of rectified potential across saidaresistance until the alternating current voltage across said rectifier input impedance exceeds a predetermined value, and additional means preventing said second diode from recti-- tying: alternating current voltages below a desired magnitude. I I

4. The invention as set forth in claim 3, wherein said first preventing means comprises a source of potential establishing upon the first rectifier cathode amore positive direct current potential than that of the first rectifier anode.

5. In a modulated carrier wave receiver, the combinaticnwith a carrier wave amplifier of the audion type, and means impressing a direct current biasupon a grid of said audion, of a diode rectifier having a cathode and a pair of anodes, a tuned input circuit having its high carrier potential terminal connected to the rectifier cathode, means coupling said input circuit to theoutput circuit of said amplifier, means grounding the low 'carrier potential terminal of said tuned input circuit, individual'resistances connecting the respective anodes to ground, capactiy by passing one of said resistances for carrier and means automatically controlling the gain of said signal amplifying means as a function of received signal energy, of means rendering said demodulator inoperative forimpressed voltages of less than a predetermined magnitude, and additional means rendering said gain controlling means inoperative for all values of received signal voltages of less than a predetermined magnitude, I

'7. In a transmission system, the combination with a signal frequency amplifier, a demodulator, an audio frequency load circuit working out of said demodulator, and means including a rectifier for automatically varying the gain of said amplifier to compensate for variations in the strength of received signals, of means for postponing cperation of said rectifier until the received signal strength exceeds a predetermined value,, and means for reducing the audio fre- I I t v t quency distortion arising from the distortion of carrier frequency and the other for audio frequency, and a sourceof direct current and adjustable means for establishing di rectbu rrent the form of the amplified signal Wave impressed upon saiddemodulator Whensaid rectifier becomes operative.

8. In a transmissionsystem, the combination with a signal frequency amplifier, a demodulator,

andan automatic gain control system operative only vvhenwthe strength of the received signal waveexceeds a predetermined value to vary the waves, said system including a rectifier which distorts the form of the amplified signal wave im- 5 pressed on said demodulator, of means for reducing the audio frequency distortion arising from said distortion of the wave form.

9. In a transmission system, the combination with a signal frequency amplifier, a demodulator, an audio frequency load circuit working out of said demodulator, and means including a rectifier for automatically varying the gain of said amplifier to compensate for variations in the strength of received signals, of means for post- 5 poning operation of said rectifier until the received signal strength exceeds a predetermined value, and condensive means for reducing the audio frequency distortion arising from the distortion of the form of the amplified signal wave impressed upon saiddemodulator when said rectifier becomes operative.

10". In combination with a signal amplifier comprising a tube having a tuned input circuit, a following tube including a cathode and at least 5? one anode and a grid electrode, means for maintaining the anode and grid electrode at different direct current potentials with respect to the cathode; a tuned input circuit connected between the cathode and anode of said following tube and including a resistor for developing a direct current voltage from signal energy'impressed on said second circuit, a control connection between an input electrode of said amplifier v and said resistor, means for connecting said grid to a point in said second tuned circuit, and a signal transmissionnetwork coupling the second tuned circuit to the amplifier output.

11. In a system as defined in claim 10, said grid connecting means being adjustable, and an audio amplifier tube having its control grid coupled to a path including said following tube grid.

12. In a system as defined in claim 10, an audio amplifier tube, and means for adjustably coupling the control grid of the latter tube tov a path including said following tube grid.

13. A rectifying system including a source of alternating current, a thermionic rectifier having a plurality of rectifying anodes connected to said source, an output circuit of said rectifier corinected to one of said anodes and including means for producing therein a rectified voltage substantially equal to the peak of the unrectified voltage applied to said rectifying system and also including a second independent output cir- 5 'cuit of said rectifier connected to another anode thereof and including therein a fixed voltage source, the rectified voltage produced in said second output circuit being effective only when the peak of the unrectified voltage applied to said rectifying system exceeds said fixed voltage.

14. A radio receiving system including a common source of signal-bearing radio frequency current and means for separating said current into a'plurality of paths, means in one path for 5 acting upon said radio frequency current so as to yield direct current for automatic volume control only when said radio frequency current exceeds a given minimum value, and means in the tron emitting cathode and two anodes, an input impedance having. one end connected to said cathode, a load circuit connected between one end of said input impedance and one of said anodes, said load circuit being responsive to the audio frequency cornponent of the demodulation products generated by said detector, a source of bias potential in series with said load circuit and making the anode to which said load circuit is connected negative with respect to said cathode, a second load circuit connected between the end of said impedance to which said first load circuit'is connected and the other of said anodes, said. load circuit being responsive to the direct current component of said demodulation products, a source-of bias potential in series with said second load circuit and making the anode to which said second load circuit is connected negative with respect to said cathode, said first a control of said carrier frequency amplifier.

16. In an electrical wave transmission system, a double diode rectifier tube having a cathode and two anodes, a single input impedance connected between the cathode and anode of said rectifier, and circuit elements connected tothe respective anodes to constitute two substantially independent output circuits for said rectifier, in combination with a source of direct current potential establishing negative bias potentials upon the respective anodes, thereby to prevent rectification in the respective output circuits until the alternating voltage across said input impedance exceeds predetermined minimum values.

17. In a modulated carrier wave receiver, the combination with a carrier wave amplifier, of a diode demodulator for demodulating the carrier waves amplified by said amplifier, and a background noise suppression device means establishing upon the diode cathode a positive bias potenfor developing a direct current voltage from said waves, means for applying said last voltage to the amplifier for regulating the gain thereof, and both said diodes being provided by a single tube having a single cathode and a pair of anodes receiving electrons therefrom.

18. In a receiving system, a tube provided with a cathode and atleast two cold electrodes, 2. tuned signal input circuit connected between the cathode and one electrode, a connection between the other electrode and said input circuit, a signal transmission tube, a gain control connection between the signal transmission tube and solely said one electrode, a demodulated signal path including a tube coupled to said other electrode, said other electrode consisting of a grid, and a direct current connection between the cathode of the last tube and a point on the circuit connecting the cathode and grid electrode,

of the first tube.

'19. In a carrier wave receiver, a carrier frequency amplifier and an. audio frequency amplifier, a detector interposed between said amplifiers and comprising within a single envelope an electron emitting cathode and two anodes, an input impedance having one end connected to said cathode, a load circuit connected between one end r 2,096,393 'of' said input impedance and one of said anodes,

said load circuit being responsive to the audio frequency component ofthe demodulation products'generatedf by said detector, a second load circuit connected between the end of said impedance to which said first load circuit is connected and the otherof said anodes, said load circuit being responsive to the direct currentcomponent of said demodulation products, a source of bias potential in series with said second load circuit and making the anode to which said load circuit is connected negative with refiers and comprising within a singleenvelope an electron emitting cathode and two anodes, an input impedance having one end connected to said cathode, a load circuit connected between one end of said input impedance and one of said anodes, said load circuit being responsive to the audio frequency component of the demodulation products generated by said detector, a source of bias potential in series with said load circuit and making the anode to which said load circuit is connected negative with respect to said cathode, a second load circuit connected between the end of said impedance to which said first load circuit is connected and the other of said anodes, said load circuit being responsive to the direct current component of said demodulation products, said first load circuit supplying voltages to actuate said audio frequency amplifier and said second load circuit supplying Voltages for automatic gain control of said carrier frequency amplifier.

PAUL O. FARNHAM. 

